The objectives of our studies are (1) to purify and characterize a nucleolar antigen which has been found in a broad range of human malignant tumors, (2) investigate the function of the antigen by initially localizing it in the nucleolus and assessing its effects on a variety of nucleolar enzymes and other components as well as its origin and (3) to evaluate the potential immunodiagnostic use of the antigen by a number of techniques. The specific aims of these studies are (1) the separation, isolation and purification of these antigens and preparation of the corresponding monospecific antibodies; (2) inasmuch as preliminary studies have indicated that the antigen(s) is localized to perinucleolar chromatin and intranucleolar chromatin in association with the fibrils and beads of the "nucleolonemas," one goal of these studies is to determine what function the antigen has in these special locations; (3) to investigate the antigen in large series of specific tumors and determine relationships of growth rates and amounts of antigen present; and (4) to determine whether the antigen is related to growth, and whether it is a fetal gene product or a product of exogenous genes.